Liquid developer for electrostatic printing



United States Patent 3,399,140 LIQUID DEVELOPER FOR ELECTROSTATIC PRINTING Frederick W. Fischer, Palos Heights, and Robert N.

Cooper, Chicago, Ill., assignors to American Photocopy Equipment Company and The Sherwin-Williams Company, jointly, corporations of Illinois and Ohio, respectively No Drawing. Filed Sept. 7, 1965, Ser. No. 485,549 4 Claims. (Cl. 252-621) This invention relates to a liquid developer useful in electrostatic photographic processes.

In the process of electrostatic photography, an electrostatic image is produced on the surface of a sensitive sheet element formed by coating a suitable substrate with a high resistance photoconducting material.

One type of sensitive sheet element useful in this process comprises a paper backing member coated with a dried layer of a liquid comprising an electrically-insulating but photoconductive dispersion of a particulate or pigmentary photoconductive zinc oxide in a non-volatile adhesive binder for the zinc oxide.

A latent electrostatic image is produced, for example, by first imparting a uniform electrostatic charge to said element by exposure to a corona discharge device. Exposing the resulting charged surface to a light image causes the electrostatic charge to dissipate in direct relation to the intensity of light radiation present in the light and shadow areas of the projected light image. Regardless of how formed, latent electrostatic images are developed by depositing finely divided opaque particles onto electrically charged areas through electrostatic attraction. The areas of latent image then assume a visible form which corresponds to the electrostatic charge image area.

Electrostatic image development utilizing the socalled dry process has been and is now extensively employed in commercial electrostatic equipment. One type of dry process electrostatic image development involves the use of finely divided opaque colored material called a toner, deposited on small glass beads or other more coarsely divided material called a carrier. The two component dry developers are generally applied by cascading the admixed dry particles across electrostatic image areas. Conventional line copying machines employing this process generally have a limited resolution power of about 125 to 250 lines per inch. When employed in continuous operations, it becomes ditficult to maintain the proper amount of toner in the two component developer composition. Thus, the dry process has a number of inherent limitations.

Another electrostatic copy development process known as powder cloud development provides the resolution necessary for improved continuous-tone development and is capable of resolutions greater than 300 lines per inch. The powder cloud is generated by agitating a mass of dry electroscopic material in a confined space, or, in the case of a liquid, by spraying through an atomizer. The particle cloud so produced is charged. The charged particles in the cloud then contact with the electrostatic charge latent image area of the copy paper. Controlling concentration of electroscopic developer particles with relation to the carrier is not a problem in the cloud process, thus the latter process lends itself particularly to continuous electrostatic copying operations and equipment.

There are, however, also a number of difiiculties involved in powder cloud development. For example, it is necessary to employ a development electrode to obtain the best resolution and solid-area coverage. The electrode must be positioned close to the latent image bearing surface area of the photoconductive sheet to operate efiiciently and it is difficult to maintain an adequate air flow 3,399,140 Patented Aug. 27, 1968 in the confined space between the electrode and the latent image bearing sheet. It is also difiicult to obtain a flow of constant particle concentrations from the powder cloud generator for extended periods of time in order to expose the latent image area of the copy sheet to a uniform powder cloud. Moreover, powder cloud particles are not all charged to the same polarity, there being a significant portion of the cloud of opposite polarity to that desired for the development of the latent electrostatic image. These oppositely charged particles tend to deposit in background areas (of less charge) making it difficult to obtain clean backgrounds and high contrast ratios.

More recently, the liquid process for development of electrostatic latent images has gained commercial importance and represents a substantial improvement over previous dry and cloud processes.

The liquid development process has been previously described by K. A. Metcalf and R. J. Wright in a paper entitled, Xerography, published in the Journal of the Oil and Colour Chemists Association, November 1956, vol. 39, No. 11, London, England, and in another paper entitled, Liquid Developers for Xerography, published in the Journal of Scientific Instruments, February 1955, vol. 32.

Liquid immersion developers (L.I.D.) employed in this newer process consist of a relatively few finely divided developer particles dispersed in a relatively large volume of insulating liquid. Liquid developers may be flowed over a surface bearing a latent electrostatic image or the surface may be immersed in a tray of liquid developer. Liquid immersion developers may also be applied by spray or roller coater.

Upon wetting the electrostatically charged surface with the developer liquid, developer particles (opaque particles not in true solution) are rapidly drawn out of the liquid and deposited onto the surface in a quantity in proportion to the density of the charged latent image areas. The development of an electrostatic image by means of liquid developer is referred to by some in the art as a form of electrophoresis. This term may be defined as the migration of charged particles suspended in a liquid toward an electrode of opposite polarity in contact with the suspension.

The liquid development process provides resolution comparable to that of powder cloud development, but avoids some of the disadvantages of dry powder development methods. For example, the electrostatic image may be easily contacted with liquid developer by any of the above mentioned means without the necessity for complicated means for obtaining uniform particle distribution during development. Developer particles obtain their charge (potential) in a liquid developer without the need for special charging equipment. Uniform dispersion of charged, opaque developer particles may be easily maintained in a liquid medium of the proper resistivity. Thus, the odds against image defects due to unequal distribution of developer particles is substantially less than in dry processes and the density of deposition of developer particles, for all practical purposes, is dependent primarily upon the electrostatic charge of the latent image. Use of a liquid having a low dielectric constant permits etfective use of a development electrode.

The liquid developers to which this invention is directed include as essential components resinous binders as hereinafter described at least in part soluble in the solvent alluded to and crystal steep asphalt in suspension.

The high resistance liquid component of the developer is an organic volatile solvent of low solvent power which is commonly expresed in terms of K.B. values of kauributanol numbers.

Solvents having a low K.B. number or value are req'uired because contrast in the ultimate print is lost if the asphalt dispersion becomes or tends to become a true solution. By choosing a solvent having a K.B. value within the range of about 20-45, only a relatively small portion of the various asphaltic fractions in the asphalt component are dissolved. The remaining fractions produce'a natural colloidal dispersion essential to the liquid immersion development system.

While not wishing to be bound by theory, it is believed that the disperse asphalt particles become sufliciently solvated or softened in the herein described hydrocarbon solvents'to promote fixing through solvent release, yet function also and are capable of 'electrophoretic deposition onto the latent charge pattern created on the surface of an electrophotographic plate or element.

While asphalt dispersions alone maybe employed as liquid developers and will develop visible images from latent image electrostatic charge images without additional reinforcement components, they are improved by inclusion with other particulate substances and binders. Visual images obtained 'with asphalt dispersions as the sole charged particulate matter exhibit the brownish color of the asphalt particles. While legible, such dispersions are not of acceptable color for commercial purposes. This deficiency may be corrected in the developer by additional inclusion of opaque and colored pigments in the developer composition.

While again not wishing to be bound by theory, it is further believed that in the developers of this invention the asphalt particles become occluded onto the surface of the pigment particles. Thus, the asphalt functions more effectively as an adhesive or binder for the pigment particles in conjunction with a rosin or rosin salt upon deposition onto surfaces carrying an electrostatic charge.

In the liquid dielectric solution of this invention containing a dispersed particulate phase as heretofore described, there is evolved a phase boundary phenomena wherein charged layers are formed at the interfacial surface between the dispersed particles and liquid phase of the developer. This phenomena gives rise to an electromotive force (zeta potential) which enables the particulate, asphalt and associated pigment to migrate (electrophoretic effect) to the charged surfaces of the electrophotographic element. Thus, it is extremely important that the non-conductive volatile organic solution, the adhesive component thereof and the dispersed phases be properly matched to obtain the optimum zeta potential [for maximum development and contrast in the final electrostatic copy.

All of the ingredients comprising the developers of this invention are believed to contribute to the control of the zeta potential of the dispersed phase developer particles in suspension. It has been found that the presence of, preferably, rosin and the soluble metal salts thereof have considerable influence upon zeta potential of the dispersed particulate phase of the liquid developers of this invention and noticeably contribute to the quality of the patent image produced by the developer.

In addition to the above mentioned essential ingredients, other components may be incorporated into the developer which aid in maintaining the desired state of dispersion of developer particles. Lecithin, for example, has been employed in small amounts without notable electrophotographic effect. Thus, its use in small amounts may be tolerated but is of questionable value because of the excellent inherent stability of the developer.

Mineral oil may be employed as an additive with some apparent beneficial effect upon the developed image characteristics. The value of the mineral oil, however, is primarily in the concentrated forms of the developer, where it is believed to serve to stabilize the suspension against undue separation by increasing the thixotropic quality.

The developer solids of this invention comprise from 5 to 50% pigment by weight. The resinous solids are from 4 550% of resins soluble in the low KB. solvent and from 5095% of asphalt by weight.

The concentration of the above developer solids with respect to high resistance liquid is extremely low in the developer when reduced to the concentration best suited to actual developing of a visible from a latent image. For example, developer solids content of the completed working developer may range-from 0.01% to'2% by weight while concentrationsof 0.05 to 0.1% are preferred. As one technician has observed, about like dirty solvent.

Replenishing solids removed from working solutions in the development process is usually accomplished'by additions thereto from considerably highernon-volatile concentrations'of stock materials. Total non-volatile concentrations by weight of stock developer may range from 1 to 6 0%. At the low end of the range of concentration of the stock solution there is insuflicient total non-volatile material available to provide ready admixtures in replenishing the working developer in active use. Above 60% nonvolatile concentration, the viscosity of the stock material is increased to an excessive level, thus making dispensing of the overly concentrated stock developer material for use in standard electrostatic copying equipment excessively difiicult. Higher solids are possilbe, but not entirely practical. Stock developers prepared according to this invention are preferably prepared at the upper non-volatile range as above recited. At concentrations above 15 to 20% level thixotropy of the system is developed which aids the suspension of the concentrated stock solution which may then be stored for a year or more without appreciable settling of the disperse particulate or particle phase.

For example, anon volatile concentration of about 50% has been found to provide a very practical and excellent product 'having non-settling properties, ample solids and satisfactory viscosity characteristics for ready reduction for use in electrostatic copying equipment replenisher devices.

The pigmentary component of this invention is not restricted to any particular color or type of pigment, but because black on white copies are preferred, carbon black pigment has'been employed herein-to illustrate the invention. Other pigmentary combinations alone, or in combination therewith, are known in the field.

The nature of the carbon black pigment employed as the pigmentary component of the developers of this invention may have a significant effect upon the quality of the visual image obtained in the development process. Sharpness of definition and black density ratio of the developed image are qualities generally affected. In relatively few instances, however, the influence of the particular carbon black pigment employed in the developer may be suflicient to reverse the charge and to cause a negative image rather than the more usual positive image to be obtained. In some instances such reversal is demanded. Generally, it has been found that the best positive image characteristics may be obtained by selecting carbon black pigments having relatively low surface area to weight ratios. For example, excellent positive image characteristics were obtained by employing carbon black pigment having a surface area of 13 square meters per gram.

A small amount of blue pigment, illustratively phthalocyanine blue, or other opaque blue dye, may also be included in the developer formulation to intensify the jetness of the black visual image obtained.

Any of the common commercial grades of asphalt may be employed in the composition. These commercial asphalt products are generally classified according to softening temperature. Low softening temperature grades are used commercially for coating flat roofs, medium for medium pitch, and steep for high pitch roofs. It is preferred to use high temperature softening material (Steep) to 200 F. for present purposes because the greater hardness of this material provides greater smudge resistance in the developed electrostatic image. These preferred asphalts are known in the trade as crystal steep asphalts. V Volatile organic liquids having high electrical resistance suitable for present purposes are also essential to the end use and include mineral spirits, odorless mineral spirits and volatile hydrocarbon or substituted hydrocarbon solvents (e.g., halogen substituents, illustratively, trichlorotrifluor'oethane; commonly known as Freon 113, having' a kauri-butanol value of from 20-45.

Odorless mineral spirits have been found to be one of the preferred solvents as the desired asphalt dispersion obtained therein and it has the additional advantage of freedom from objectionable odors characteristic of many aliphatic hydrocarbon solvents otherwise useful. Dissolution of the disperse phase asphalt particles is objectionable as it interferes with the function of the asphaltic component in the 'L.I.D. composition in use. Moreover, dissolving the asphalt particulate phase places additional asphalt 'color bodies in solution which may cause staining of porous surfaces by absorption and mechanical entrapment. Asphalt staining resulting from the use of high solvency solvents has been observed in developers of this invention producing noticeably darker (normally white) background areas of the developed electrostatic image (reduced contrast ratios) particularly where the surface upon which the image is formed is of the zinc oxide containing type. I

The high resistance solvent is preferably of relatively high volatility to facilitate drying of the sheet element subsequent to development of the electrostatic image. Kerosene, for example, is within the specified solvency range but is not preferred for use in electrostatic copying equipment because of its slow evaporation rate. Rapid drying of the developed sheet element under machine operating conditions is excessively retarded.

The evaporation rates of aliphatic hydrocarbon solvents vary over a wide range. Kerosene with an evaporation rate of 325 minutes represents the low limit of the range while hexane with an evaporation rate of less than one minute is one of the most volatile solvents in the useful group. These evaporation rates express the time required for 0.5 cc. of solvent to evaporate from filter paper and are relative values. Despite their relativity, the rates are, nonetheless, useful as definitive means. Evaporation rate will vary considerably according to atmospheric conditions (temperature, humidity and barometric pressure).

Odorless mineral spirits with a relative evaporation rate of about 60 minutes represents a useful solvent having a low volatility rate. However, flash points must also be considered as a limiting factor with respect to useful solvents having a relatively high volatility rate. Explosion dangers increase as the flash point is decreased.

The Red Label solvents (those whose fl-ash point requires a red label by law) are to be avoided for present purposes. Their use in electrostatic copying equipment presents a definite fire hazard. Red Label solvents are defined as having a flash point below 80 F. as measured by the Tag Open Cup method of test.

The fourth essential component of the developer composition is a resin showing at least partial solubility in the low K.B. solvent. Rosin or metal salts of rosins are preferred examples. It is preferred to employ grades of these materials which are substantially free of matter unsoluble in aliphatic hydrocarbons of the low K.:B. value as previously described.

'For example, excellent results were obtained by employing in the invention combinations a commercial grade of zinc resinate containing 8.9% zinc and having a practically zero acid value.

The L.I.D. concentrates of this invention may be prepared by various means including high shear mixing. However, it is preferred to obtain a higher regree of dis persion of pigment particles than may be obtained by simple mixing techniques because finely divided pigment particles remain more readily in suspension and are of higher hiding quality. Improved stability characteristics of the completed developer are the result of better dispersions. In addition, graininess and resolution of the visual image obtained with the developer are improved by reducing the size of the disperse phase developer particles.

Dispersing equipment commonly employed in the coatings industry may be employed. If one desires, all of the ingredients may be charged into a ball mill or pebble mill holding out a portion of the solvent which may be later added to the dispersion as a mill wash after milling has been completed. Other types of dispersing equipment may be used by varying the amounts of solvent, etc., employed in accordance with the particular type of equipment chosen and in accordance with the techniques well known in the protective and decorative coating and ink-making arts. It may, for example, be found preferable to disperse the asphalt in a portion of the solvent prior to the grinding operation. This may be conveniently accomplished by preparing a stock base of about 50% solvent and 50% asphalt. One common method is by heating and stirring the mixture at 250-300 F. until uniform dispersion of the asphalt in solvent is obtained.

From the foregoing, it is evident that the novel developers of this invention are subject to considerable formulative variation. The following examples are intended as illustrative but not exhaustive of useful forms of the invention.

EXAMPLE I Developer concentrate Parts by weight Crystal steep asphalt 9.03 Zinc resinate 3.87 F. T. carbon black 3.87 Nigrosine base B (deep blue aniline dye) 1.94 Odorless mineral spirits 81.29

The above ingredients were charged into a ball mill and ball milled for 8 hours. The degree of dispersion obtained was 7H as measured on the Hegman Grind Gauge.

EXAMPLE II Developer concentrate Parts by weight The above ingredients were ball milled for 8 hours after which time a dispersion of 7H was obtained.

1 EXAMPLE III A working liquid developer composition of approximately 0.1% developer solids concentration was prepared by mixing 5 parts by weight of the developer concentrate prepared in Example I with 1000 parts of odorless mineral spirits. A charged latent image containing zinc oxide coated paper sheet was submerged therein and the visual image formed with good contrast and little background coloration on the sheet.

EXAMPLE IV A working liquid developer composition of approximately 0.1% developer solids concentration was prepared by mixing 2 parts by weight of the developer concentrate prepared in Example II with 1000 parts of odorless mineral spirits. Upon testing by spraying, a good visual image copy was obtained of the latent image charged onto the zinc oxide paper.

EXAMPLE V Developer concentrate Parts by weight Crystal steep asphalt 9.0

The above ingredients were ball milled for 8 hours after which time the degree of dispersion obtained was 7H as measured by the Hegman Grind Gauge.

EXAMPLE VI A working developer composition of approximately 0.3% developer solids by weight was prepared by mixing 4.6 gm. of the developer concentrate prepared in Example V with 2,000 cc. of odorless mineral spirits. A charged latent image containing zinc oxide coated paper sheet was submerged therein and the visual image formed with good contrast and little background coloration of the sheet. However, black density of the developed image was less than Examples III and IV.

What is claimed is:

1. A process for developing a latent image on an electrostatically charged surface which comprises: con-v tacting the charged surface with a liquid developer containing finely divided developer solids in a liquid component; said liquid component comprising a volatile highresistivity aliphatic hydrocarbon or halogenated hydrocarbon solvent having a kauri-butanol value between about 20 and 45; and said developer solids consisting essentially of about 50 to 95 weight percent binder solids and.,about 5 to 50. pe r cent pigment solids;..said binder, solids consisting essentially of about to 95 percent asphalt and about 5 to 50 percent of binder resin selected from the group consisting of rosin, metal salts of rosin and mixtures of rosin and metal salts of rosin.

2. The process of .claim 1 wherein, the liquid compqnent consist essentially of aliphatic hydrocarbons having a-fiash point above F.,, the pigment solidsinclude carbon black having a surface area oiabout 13- square meters per gram, and' thetotal developersolids content is about 0.01 to 2 weight percent ofthe liquid developer.

3. The process of claim 2 wherein the binder solids include crystal steep asphalt.

4. A liquid developer composition for-electrostatic printing consisting essentially of 0.01 to 2 weight percent developer solidsin an aliphatic hydrocarbon or halogenated hydrocarbon solvent having'a kauri-butanol value of about 20 to 45 and low dielectric constant;

said developer solids containing about 5 to 50 weight percent pigment particles and about 50 to percent binder -particles, 5 to 50=percent of thebinder comprising a resin selected from the group consisting of rosin, calcium salt of rosin, zinc salt of rosin,

and mixtures of the rosin and salts, said binder comprising 50 to 95 percent of an asphalt substantially insoluble in the organic liquid.

References Cited UNITED STATES PATENTS 2,297,691 10/1942 Carlson 25262.1 2,618,522 11/1952 Wise 252-621 X 2,649,381 8/1953 Hempel et a1. 106234 X 2,877,133 3/1959 7 Mayer 252-62.1 X 3,165,420 1/l965 Tomanek et al. 25262.1 X

LEON D. ROSDOL, Primary Examiner.

I. D. WALSH, Assistant Examiner. 

1. A PROCESS FOR DEVELOPING A LATENT IMAGE ON AN ELECTROSTATICALLY CHARGED SURFACE WHICH COMPRISES: CONTACTING THE CHARGED SURFACE WITH A LIQUID DEVELOPER CONTAINING FINELY DIVIDED DEVELOPER SOLIDS IN A LIQUID COMPONENT; SAID LIQUID COMPONENT COMPRISING A VOLATILE HIGHRESISTIVITY ALIPHATIC HYDROCARBON OR HALOGENATEAD HYDROCARBON SOLVENT HAVING A KAURI-BUTANOL VALUE BETWEEN ABOUT 20 AND 45; AND SAID DEVELOPER SOLIDS CONSISTING ESSENTIALLY OF ABOUT 50 TO 95 WEIGHTS PERCENT BINDER SOLIDS AND ABOUT 5 TO 50 PERCENT PIGMENT SOLIDS; SAID BINDER SOLIDS CONSISTING ESSENTIALLY OF ABOUT 50 TO 95 PERCENT ASPHALT AND ABOUT 5 TO 50 PERCENT OF BINDER RESIN SELECTED FROM THE GROUP CONSISTING OF ROSIN, METAL SALTS OF ROSIN AND MIXTURES OF ROSIN AND METAL SALTS OF ROSIN. 